Protection for semiconductor power diodes



April 11, 1961 E. SALZER 2,979,644

PROTECTION FOR SEMICONDUCTOR POWER DIODES Filed May 13, 1960 Fig.l

I I 7 I2 I20 8 IN VEN TOR.

NA/WW ki/W PROTECTION FOR SEMICONDUCTOR POWER DIODES Erwin Salzer,Waban, Mass, assignor to The Chase- Shawmut Company, Newburyport, Mass.

Filed May 13, 1960, Ser. No. 29,000

7 Claims. (Cl. 317-234) This invention is concerned with semiconductorrectifiers, and more particularly with large rectifiers of thatdescription comprising numerous semiconductor rectifier cells connectedin parallel into electric circuits.

Some semiconductor rectifiers such as, for instance, rectifierscomprising germanium cells, or silicon cells, or other rectifier cellsinvolving a relatively small mass and intended to operate at relativelyhigh current densities requires co-called cell fuses to protect soundcells from the over-currents resulting from the breakdown of any givencell in such a rectifier. Cell fuses must be current-limiting, mustperform a very critical task and must, therefore, be precisionengineered and carefully constructed.

Each cell in a semiconductor rectifier has a predetermined damage 1which is a constant value, and each cell fuse must be designed andconstructed in such a way that the clearing 1 thereof is less than thedamage z -t of the cell to be protected by the fuse. This coordinationof f- -t values is knownas matching of the cell and the cell fuse. Ifthere is a mismatch of cell and cell fuse the fuse may blowunnecessarily and thereby impair the continuityof service, or the cellfuse may fail to blow when it should blow and be responsible fordestruction of a smaller or largernumber of cells likely to result ininterruption of service. It is thus apparent that perfect matching ofthe constituent cells of a rectifier and of the cell fuses thereof is amatter of crucial importance.

The behavior of a cell fuse is characterized by its fusing z -t and itstotal clearing i -t. For practical purposes a fixed ratio may be assumedto exist between these two values. fusing or melting i of a given cellfuse must be less than the damage 9-: of the rectifier cell to beprotected by the cell fuse.

The damage i -t of the cell as well as the fusing z' -t of the cell fusedepend upon the initial temperature of the two devices, i.e. uponwhether the two devices are initially hot or cold, and on ambienttemperature conditions. Hence any given damage and fusing z' t valuemust expressly, or impliedly, refer to an initial temperature condition.a

It occurs quite frequently in connection with conventional semiconductorrectifier structures that the cells and the cell fuses are initiallyperfectly matched or balanced but become subject to different heatingand cooling conditions when built into and forming an integral part ofthe rectifier and operating inside of the rectifier.

It is, therefore, one object of this invention to provide means whichtend to preclude a relative change of the damage i -r of the cells andof the fusing 1 -2 of the cell fuses when operated inside of a rectifierstructure.

Another object of the invention is to provide means precludingunnecessary blowing of cell fuses and precluding failure of cell fusesto blow when they ought to Both the total clearing 1 -1! and the2,979,644 Patented Apr. 11, 1961 blow in order to protect the cells forthe protection of i which they are intended.

Another object of the invention is to provide means precludinginterruptions of service resulting from failure of cell fuses to performas such fuses are supposed to perform.

Large semiconductor rectifiers are generally provided with convectioncooling systems such as forced air cooling systems or circulating liquidcooling systems. If

these cooling means maintain the temperature of the cells relativelyfixed irrespective of the load current they carry and if these coolingmeans fail to cool the cell fuses in a fashion similar to their coolingaction upon the cells, then the initial match of the damage i -t of thecells and of the fusing 1 of the cell fuses becomes ineffective, and thecell fuses tend to blow prematurely, i.e. under load conditions which donot warrant blowing thereof.

It is, therefore, another object of the invention to pro-v videconvection cooled semiconductor rectifiers wherein Cell fuses havefusible elements of silver, because silver combines a relatively highconductivity with a relatively small fusing i -dt. fusible elements ofsilver are provided with an overlay of a low fusing point metal adapted,on fusion thereof, to sever the base metal by a metallurgical reactionor alloying between the oveflay metal and the base metal. Such cellfuses have been'described in considerable detail in US. Patent 2,921,250to K. W. Swain, Coordinated Static Power Rectifiers and Current-LimitingFuses, issued January 12, 1960, and reference may be had to this patentfor additional details on this type of cell fuses. It is desirable, inorder to achieve the time current characteristic best adapted foreffective cell protection, to apply for cell protection fuses having afuss ible element which is coated wholly or in part with a low fusingpoint metal. This, however, greatly reduces the temperature differencebetween normal operating temperature and blowing temperature. As aresult of that reduction of temperature range changes in ambient tem-'perature are likely to affect more critically the operation of cellfuses having fusible elements or fuse links of this character than theoperation of cell fuses whose links are made of silver and lack anyoverlay of an alloy forming link-destroying low fusing point metal.

It is, therefore, another object of this invention to provide means forcompensating the relative sensitivity of cell fuses having fusibleelements comprising overlays of the aforementioned character to ambienttemperature conditions, i.e. ambient cooling or heating conditions.

Further objects and advantages of the invention will become apparent asthis description proceeds, and the features of novelty whichcharacterize the invention will be pointed out with particularity in theclaims annexed to, and forming part of, this specification.

For a better understanding of the'invention reference may be had to theaccompanyind drawing wherein Fig. 1 shows a cross-section of a hollowwater cooled bus structure supporting a semiconductor rectifier cell anda cell fuse;

Fig. 1a is a section along 1-1 of Fig. 1; and

Fig. 2 refers to a modification of the structure of Fig. 1 and shows awater cooled bus structure supporting a semiconductor rectifier cell anda cell fuse in a fashion somewhat different from that shown in Fig. 1.

Large semiconductor rectifier structures comprising convective coolingmeans have been described in numerous publications and therefore do notneed to be described here. For information on that subject reference'maybe In one type of cell fuses the had to such publications, eg.Transaction Paper 58-219 of The American Institute of ElectricalEngineers, by

I. K. Dortort, Current Balancing Reactors for Seniicon ductorRectifiers. The drawings and the following description are limited tothe changes made in conventional semiconductor rectifier design.

Referring now to the drawings, and more particularly to Figs. 1 and 1athereof, reference character B has been applied to generally indicate ahollow liquid-cooled bus bar,.reference character C has been applied togenerally indicate a semiconductor diode and reference character CF. hasbeen applied to generally indicate a currentlimiringcell fuse. The busbar C comprises duct-defining means 1 adapted to contain a body offlowing cooling liquid 2. Front plate In of bus 1 has a plurality ofopenings 3 of which but one is shown'in' Fig. 1. Opening 3 is covered bya plate 4 provided with cooling fins 4a. The front plate In. and thecover plate 4 are provided with registering holes 115, 4b for insertingscrewthreaded bolts (not shown) for screwing plate 4 against plate In. Agasket 5 is arranged between front plate In and cover plate 4 topreclude cooling liquid inside. of bus B from leaking out, betweenplates 1a and 4. Plate 4 forms an internally screw-threaded recess 40.The semi-conductor power diode C may be a silicon diode, e.g. a highcurrent silicon 4JA60-4JA61 cell. Such diodes comprise a pair .ofterminals of which one is generally formed by a screw-threaded plug andthe other by a braid. The silicon diode shown in Fig. 1 is provided withtwo terminals in the form of screw-threaded plugs 6, 7. Plug 6 isscrewed into recess 40 formed by plate 4. The current-limiting cell fuseCF comprises a casing 8 of insulating material closed on both ends byterminals in the form of plugs 9, 10 held in position .by transversesteel pins 11. The axially inner surfaces of plugs 9, 10 are grooved andthe fusible element 12 is inserted into these grooves with the axiallyouter ends thereof. A body 12a of quartz sand surrounds the ins ibleelement 12. Fusible element 12 is formed by a ribbon of silver havingtwo lateral V-shaped incisions defining a point of restrictedcross-section which forms a point heat source when ribbon 12 is carryingcurrent. The entire surface of. ribbon 12 may be "tin plated for reasonsset forth in detail in the above referred topatant to K. W. Swain. Theleft plug terminal 9 isrprovided with a screw-threaded recess 9a intowhich the screw-threaded plug connector '7 of power diode C en ters. Theright plug terminal 10 is provided with a screw-threaded recess 10a intowhich a hex screw '13 enters clamping lug 14 provided with a braid 15against the axially outer surface of plug 10. To facilitate heatexchange between cell C and the coaxial cell fuse CF the diameter ofscrew-threaded connector plug'7 which enters into the internallyscrew-threaded recess 9a of plug 9 must be relatively large. Parts 7, 9integrate cell'C and current-limiting cell fuse CF into a thermal unitor stack which is characterized by its ability to readily exchange heatbetween the constituent elements thereof.

In the structure of Figs. 1 and 1a the operating temperature of the cellfuse CF is directly or immediately affected by the cooling action towhich the power diode .C is subjected, and the relation between thetemperature of the cell fuse CF and that of the cell C remainsubstantially constant. The relation between pre-heating of the powerdiode C and that of the cell fuse CF remaining substantially constant,the initial match of the i -t values of both circuit elementsremains-effective, and thus the cell fuse keeps on properly performingits function irrespective of changes in ambient conditions includingchanges in the cooling action of the coolant flow- Therefore there is noneed of describing again the bus structure B shown in Fig. 2. Thearrangement of Fig. 2 includes a cell fuse CF and a cell C. The cellfuse CF comprises a tubular casing 8 closed on both ends thereof byterminal plugs 9, 10, held in position by transverse steel pins -11 andconductively interconnected by silver ribbon 12 defining, a-point ofdrastically reduced cross-section. Plug 9 is provided with an internallyscrewthreaded recess 9a into which the right screw-threaded end of aplug connector. 16 enters, The left screwthreaded end of plug connector16 is screwed into the internally screw-threaded recess 40 ofv coverplate 4. Plug 10 is provided with an internally screw-threaded recess10a into which the screw-threaded terminal plug 6 of power diode Centers. The other terminal of power diode C is in the form of a braid17. The link or fusible element of cell fuse CF is 'submersed in a body12a of quartz sand which has .a relatively high thermal conductivity andtends to readily transfer the heat generated in power diode C to coverplate 4 and to the cooling liquid flowing past the cooling fins 4 1 oncover plate 4-.

Itwill be understood that I have illustrated and described herein twopreferred embodiments of my invention and that various alterations maybe made therein without departing from. the spirit and scope of theappended claims.

I claim as my invention:

1. A rectifier assembly comprising a semiconductor rectifier cell havinga pair of terminal elements, a current-limiting cell fuse having a pairof terminal elements and being arranged in coaxial relation to saidcell, one of said pair of terminal elements of said cell and one of saidpair of terminal elements of said cell fuse being directly joined byscrew-thread means of relatively large diameter forming integral partsof said one of said pair of terminal elements of said cell and of saidone of said pair of terminal elements of said cell fuse, saidscrew-thread means being adapted to promote heat exchange betweensaidcelLand said cell fuse and'to integrate said cell and said cell fuseinto a thermal unit.

2. A rectifier assembly comprising a semiconductor rectifier cell havinga pairof terminal'elements including a screw-threadedconnector plug; acurrent-limiting cell fuse including a tubular casing, a. pairofgterminal plugs plugging the ends of said casing, a conductorincluding a portion of silverconductively. interconnecting said pair ofterminal plugs and an overlay of an alloy-forming metal having asubstantially lower fusing point than silver on said portion. of saidiconductor; one of said pair of terminal plugs :having an internallyscrew-threaded recess receiving said screw-threaded connector plug.

3. A rectifier assembly comprising a semiconductor rectifier cell; acurrent-limiting cellfuse including a tubular insulating casing arrangedincoaxial relation to said cell, a pair of terminal plugs closing-saidcasing and a fusible element conductively interconnecting said pair ofterminal plugs; a first pair of cooperating screw-threaded male andfemale surfaces one on saidcelland one of said pair of plugsestablishing an electric series connection between said cell and saidcell fuse and uniting said cell and said cell fuse into an integralstack structure, iOint fiuid'cooling means for said cell and said cellfuse arranged on one end of said stack structure, and a secondpair-ofcooperating screw-threaded male and female surfaces'for attach- ,ingsaid stack st'ructure' to-said joint fluid cooling means.

4. A rectifier assembly comprising a semiconductor rectifier cell havinga pair of coaxial screw-threaded connector plugs; a fluid-cooled hollowbus having a female screw-threaded surface cooperatively engaged by oneof said pair of connector plugs of said'cell; a current-limiting cellfuse including a tubular insulating casing arranged in coaxial relationto said .e ell and .to said pair of connector plugs thereof, a'pairvofterrnirial plugs closing said casing and fusible element conductivelyinterconnecting said pair of terminal plugs; and one of said pair ofterminal plugs of said cell fuse having an internally screwthreadedrecess cooperatively engaged by the other of said pair of connectorplugs of said cell.

5. A rectifier assembly comprising a semiconductor rectifier cell havinga screw-threaded plug connector; a current-limiting cell fuse includinga tubular insulating casing arranged in coaxial relation to said celland to said plug connector thereof, a pair of terminal plugs closingsaid casing and a fusible element conductively interconnecting said pairof terminal plugs, one of said pair of terminal plugs having ascrew-threaded plug connector and the other of said pair of terminalplugs having an internally screw-threaded recess cooperatively engagedby said plug connector of said cell; and a fluid-cooled hollow bushaving a recessed screw-threaded surface cooperatively engaged by saidplug connector on one of said pair of terminal plugs of said cell fuse.

6. A rectifier assembly comprising a semiconductor rectifier cell havinga screw-threaded plug connector; a current-limiting cell fuse includinga tubular insulating casing arranged in coaxial relation to said celland to said plug connector thereof, a pair of terminal plugs closingsaid casing and a fusible element conductively interconnecting said pairof terminal plugs, one of said pair of terminal plugs having ascrew-threaded plug connector and the other of said pair of terminalplugs having an internally screw-threaded recess cooperatively engagedby said plug connector of said cell; a fluid-cooled hollow bus definingan opening, and a cover plate for said opening having an inner surfaceand an outer surface, said inner surface forming a system of coolingfins and said outer surface defining an internally screw-threaded recesscooperatively engaged by said plug connector on one of said terminalplugs of said cell fuse. s

7. A self-protected rectifier unit comprising a semiconductor rectifiercell having a screw-threaded plug connector; a current-limiting cellfuse including a tubular insulating casing arranged in coaxial relationto said cell and to said plug connector thereof, a pair of terminalplugs closing said casing and a fusible element conductivelyinterconnecting said pair of terminal plugs; one of said pair ofterminal plugs defining an internally screwthreaded recess cooperativelyengaged by said screwthreaded plug connector of said cell.

No references cited.

